1. Field of the Invention
The present invention relates to a wavelength conversion method by four wave mixing, a wavelength conversion device and a nonlinear fiber suitable therefor.
2. Discussion of Background
Various optical signal processing techniques employing a nonlinear fiber having a high third-order nonlinear coefficient (γ) have been developed, and as one of these techniques, wavelength conversion by four wave mixing (hereinafter referred to as FWM wavelength conversion) may be mentioned.
FWM wavelength conversion is to make a signal light having a wavelength λS and an intensity IS and a pump light having a wavelength λP (provided that λP<2λS) and an intensity IP (provided that IP>IS) enter a nonlinear fiber having a length L and an absorption coefficient α and to generate a converted light having a wavelength λC of λSλP/(2λS−λP) and an intensity IC, and the wavelength conversion efficiency (η) i.e. IC/IS is in proportion to <γ×IP×{1-exp(−α×L)}÷α>2.
For FWM wavelength conversion, usually a silica nonlinear fiber is used as the nonlinear fiber, and for example, η of −28 dB at IP=5 mW and L=4.43 km has been reported (Kimiya Kawanami et al, Fiber Nonlinear Coefficient Measurement Based on FWM Method with Depolarized Pump Light, 29th European Conference on Optical Communication Proceedings, September 2003, Vol. 3, p. 552-553).
γ (hereinafter γ represents a third-order nonlinear coefficient for a light having a wavelength of 1,550 nm) of the silica nonlinear fiber for a light having a wavelength of 1,550 nm is typically from 10 to 25 W−1km−1, and in order that η is higher, use of a nonlinear fiber with a higher γ is considered. As such a nonlinear fiber, one made of bismuth oxide glass has been known (for example, JP-A-2001-213640).
The above η is usually required to be at least −40 dB, and as described above, as FWM wavelength conversion which satisfies this requirement, one employing a silica nonlinear fiber has been known. However, the FWM wavelength conversion has had such problems that it is not easy to find the fiber breakage portion and a compact wavelength conversion device is hardly obtained, since the silica nonlinear fiber has a long L.
On the other hand, if it is attempted to decrease L by using a nonlinear fiber made of the above bismuth oxide glass instead of the silica nonlinear fiber, since the transmission loss of the nonlinear fiber is typically so large as at least 1 dB/m, the signal light, the pump light or the wavelength-converted light (converted light), particularly the pump light tends to be attenuated while it is transmitted through the nonlinear fiber, whereby no desired q tends to be obtained.